The prior art discloses heat-sealable polyethylene film which can be made into disposable pouches for packaging liquids and other flowable materials. These pouches are commonly used as consumer packages for containing milk, particularly shelf-stable sterilized milk. The filled pouches are commonly stacked in a larger outer container, such as an open-topped carrier, crate, or display rack, so each stack is several pouches deep. The carriers can also be stacked. Then the stacked carriers are transported by truck, rail, or some other form of transportation to a store. The ultimate consumer selects individual pouches from the carrier in the store.
A polyethylene pouch as consumer packaging for milk and other liquids has advantages over containers used in the past: glass bottles, paper cartons, and high density polyethylene jugs. Compared to these alternative containers, a polyethylene pouch consumes less raw material, requires less space in a landfill, can be recycled, can be processed easily, requires less storage space, uses less energy for chilling (as when the shelf-stable milk is chilled for the first time by the consumer) because it transfers heat readily, and can be safely incinerated. A polyethylene pouch also can be reused. For example, the empty pouch can be used as a freezer bag, sandwich bag, or as a general purpose storage bag.
A composition containing a very low density linear ethylene material in at least the seal layer of a pouch is disclosed in U.S. Pat. No. 5,364,486, which is incorporated here by reference in its entirety. This material is described in that patent as "comprising (a) from 10 to 100 percent by weight of at least one polymeric seal layer of a very low density linear ethylene copolymer interpolymerized from ethylene and at least one alpha-olefin in the range of C.sub.3 -C.sub.10 and having (1) a density of from about 0.89 g/cm.sup.3 to less than 0.915 g/cm.sup.3, (2) a melt index of less than about 10.0 g/10 minutes and (3) (i) a hot tack or heat seal initiation temperature of less than 100.degree. C. at a force of at least 1N/inch (39.4N/m) or (ii) achieving a hot tack strength of at least 1 N/inch (39.4N/m) at a seal bar temperature of about 110.degree. C. at less than 0.2 seconds using the Hot Tack Strength Method or achieving a heat seal strength of at least 1 lb.f/inch (175N/m) at a seal bar temperature of about 110.degree. C. at less than 0.25 seconds using the Heat Seal Strength Method; and (b) from 0 to 90 percent by weight of at least one polymer selected from the group consisting of a linear copolymer of ethylene and a C.sub.3 -C.sub.18 -alpha-olefin having a density of greater than 0.916 g/cm.sup.3 and a melt index of from 0.1 to 10 g/10 minutes, a high-pressure low density polyethylene having a density of from 0.916 to 0.930 g/cm.sup.3 and a melt index of from 0.1 to 10 g/10 minutes and ethylene-vinyl acetate copolymer having a weight ratio of ethylene to vinyl acetate from 2.2:1 to 24:1 and a melt index of from 0.2 to 10 g/10 minutes."
One unresolved problem in the art is the relatively high occurrence of "leakers"--pouches which develop leaks. Leakers are manifested both as the pouches are packed and due to later handling. Leakers which are not discovered and removed at the time of packaging are a particular problem because the pouches are transported in stacks. Liquid which has escaped from a single leaking pouch in a stack can soil the leaking pouch, as well as several or even all of the intact pouches in the stack. If the liquid is sticky (like fruit juices) or subject to spoilage on exposure to air (like shelf-stable milk), the soiled pouches can develop a disagreeable feel or odor. Consumers may avoid buying many stacked pouches which have been soiled by one leaker. It is thus important to minimize the number of pouches which develop leaks on the way to the consumer.
In particular, prior art films made into pouches have a high incidence of defects which develop in the material of the pouch, allowing flowable material such as milk to escape from the pouch.
Leakers have largely been attributed to the film used to make the polyethylene pouches known in the prior art, and particularly its burst strength, puncture strength, and seal strength. Prior efforts to reduce the incidence of material fatigue crack defects have therefore focused on these properties of the film. One important cause of material fatigue crack formation, detailed below as part of the present development, is believed to have escaped the notice of prior investigators.
The M, M(2) and M(1.3) tests are pouch performance tests described in U.S. Pat. No. 4,521,437. These "M" tests are carried out by dropping a sample of filled pouches from a defined height (stated in meters in parentheses: for the M(2) test the height is 2 meters) to determine what percentage of them will break when dropped.
One sealant film disclosed in the prior art has been described as providing pouches with a smaller (better) M-test value than that obtained for prior pouches. More specifically, the pouches have been described as providing an M(1.3)-test value of less than about 5% (breakage) for pouches having a volume of from 0.1 to 1.3 liters.
Although the performance of the prior art films has been satisfactory in many respects, the industry desires a film having better performance, and particularly fewer leakers, when it is fabricated into hermetically sealed pouches containing flowable materials.